1. Field of the Invention
The present invention relates to an optical transmission system and a method of controlling an optical transmission system, and more particularly to a method of controlling an optical transmission system for compensating for chromatic dispersion that is caused when an optical signal is transmitted through an optical fiber, and an optical transmission system for transmitting an optical signal through an optical fiber while compensating for chromatic dispersion.
2. Description of the Related Art
In recent years, the WDM (Wavelength Division Multiplex) system for multiplexing light rays having different wavelengths to simultaneously transmit a plurality of signals through a single optical fiber has been used as a technology for high-speed large-capacity optical transmission.
Light is transmitted through an optical fiber at different speeds for different wavelengths (different wavelength components based on the spectrum of light). Therefore, when a pulse of light is transmitted over an increased distance, it suffers chromatic dispersion which deteriorates the pulse waveform. Chromatic dispersion represents the difference between the propagation times of two monochromatic light waves whose wavelengths differ 1 nm from each other, and is expressed in a unit of ps/nm. A dispersion coefficient represents the difference between the propagation times of two monochromatic light waves whose wavelengths differ 1 nm from each other when the monochromatic light waves are propagated for 1 km, and is expressed in a unit of ps/nm/km. For example, an SMF (Single Mode Fiber) that is usually used as an optical fiber has a dispersion coefficient in the range from 15 to 16 ps/nm/km in the vicinity of 1.55 μm. When an optical signal having a wavelength band width of 0.1 nm in the vicinity of 1.55 μm is applied to an SMF having a length of 100 km, a time difference ranging from 150 to 160 ps is developed between the longer and shorter wavelength sides of the wavelength band width at the exit end of the SMF, indicating that the signal waveform is distorted.
If the waveform of a transmitted optical signal is deteriorated by chromatic dispersion in the WDM system for high-speed large-capacity optical transmission, the characteristics of the received optical signal are greatly degraded, adversely affecting the WDM system. It is customary to compensate for the chromatic dispersion caused by the optical fiber by adding the same quantity of chromatic dispersion having an opposite sign to equivalently eliminating or canceling the chromatic dispersion. A process of performing such chromatic dispersion compensation in each span of the optical transmission path, e.g., between an optical signal transmitting terminal station and an optical signal repeater, between optical signal repeaters, and an optical signal repeater and an optical signal receiving terminal station, for designing the chromatic dispersion compensation over the optical transmission path from the optical signal transmitting terminal station to the optical signal receiving terminal station is referred to as dispersion management.
According to the dispersion management, dispersion compensation modules (DCMs) are disposed before or after repeaters on the transmission path and transceivers in the terminal stations, for compensating for chromatic dispersion produced on the transmission path.
FIG. 22 of the accompanying drawings is a diagram showing a dispersion map. The dispersion map represents a transition of accumulated residual dispersion on the vertical axis with respect to the distance on the horizontal axis. FIG. 22 shows a dispersion map M1 of accumulated dispersion in a central channel 40 when a WDM system 100 transmits 80 light waves that are multiplexed by way of WDM.
The WDM system 100 is a system for performing WDM transmission from a transmitting station 101 to a receiving station 102 through a transmission path in the form of an SMF as an optical fiber which has repeaters 103, 104 at equal repeating intervals or spans.
Since the SMF has a positive dispersion value, the transmitting station 101, the receiving station 102, and the repeaters 103, 104 have respective DCMs 105-1 through 105-4 having a negative dispersion value, e.g., modules constructed of dispersion compensation fibers (DCFs) having a negative dispersion value which are assembled as coils in a length to provide a desired dispersion value, for performing dispersion compensation. The DCM disposed in the transmitting terminal station is referred to as a DCT (Dispersion Compensation Terminal).
Even though the DCMs are used to compensate for chromatic dispersion, the waveform of a transmitted optical signal is distorted to degrade the transmission characteristics due to a nonlinear effect such as SPM (Self Phase Modulation) that is produced principally in the transmission path of the system. Therefore, a certain limit (an upper limit) is posed on the optical power of a signal that is applied to the transmission path. Typically in a system for amplifying and repeating an optical signal, reducing the optical power of the signal deteriorates the SNR (Signal to Noise Ratio) of the signal. Consequently, there is also a limit posed on the reduction of the optical power, i.e., a lower limit on the optical power. Under these circumstances, it has been the general practice to apply a constant value of optical power to the transmission path and the DCMs within a certain limitative optical power range which is established on the assumption that the span distances are equal to each other over the transmission path.
Similarly, the same value of optical signal input power (transmission path input power) within a certain limitative optical power range is applied to every span of the optical fiber transmission path, and the same value of optical signal input power (DCM input power) within a certain limitative optical power range is applied to every one of the DCMs 105-1 through 105-4.
A review of the dispersion map M1 indicates that the positive dispersion in the central channel over each span of the SMF is compensated for by the negative dispersion of the corresponding one of the DCMs 105-1 through 105-4.
The chromatic dispersion of the optical signal having the central wavelength has been described above. Actually, however, since a WDM signal made up of multiplexed wavelengths is transmitted through a transmission path, the dispersion caused by the transmission path differs from wavelength to wavelength. Stated otherwise, the chromatic dispersion depends on the transmitted wavelength, a property referred to as a dispersion slope, and the chromatic dispersion characteristics are different in all the channels, i.e., the dispersion slope is not flat.
A quantity of accumulated dispersion up to the receiver is called residual dispersion (RD), and a range of accumulated dispersion within a dispersion deterioration penalty range that is allowed by the system is called RD tolerance.
If a chromatic dispersion value deviates from the RD tolerance, then it cannot be ensured that the reception side can identify signals “0”, “1” from each other, i.e., the deterioration of the eye aperture becomes so large that it is difficult to identify the data. Therefore, the WDM system is required to perform dispersion management such that all the multiplexed wavelengths or channels fall in the RD tolerance.
It has been proposed to compensate for chromatic dispersion with dispersion compensation modules having step-like dispersion values in an optical transmitter, an optical receiver, and optical repeaters, and to compensate for chromatic dispersion in an optical fiber transmission path with an optical phase conjugate unit (see, for example, Japanese unexamined patent publication No. 07-154324 (paragraph Nos. [0020] through [00281], FIG. 4)).
According to the conventional dispersion management as described above, it is assumed that DCMs are disposed at equal intervals on the optical fiber transmission path for performing dispersion compensation at equal spans, and a system is designed by giving the same value of transmission path input power within the limitative power range to every span of the optical fiber transmission path and giving the same value of DCM input power within the limitative power range to every DCM. Furthermore, inasmuch as the wavelength deterioration due to a nonlinear effect is greater over a longer distance, dispersion management has been applied to a path over a longest distance within the network.
In actually constructed systems, however, optical repeaters cannot be positioned at equal spans, and the spans have different distances and hence cause different losses. Consequently, each of the spans is not optimized for OSNR (Optical S/N Ratio) and nonlinear quantity. Moreover, the conventional system designing process is problematic in that if a WDM system is constructed as a network including an OADM (Optical Add Drop Multiplex) node or a HUB node in order to optimize itself for a dispersion map of a long-distance through path, a path interconnecting terminal stations, the transmission performance is greatly deteriorated with respect to new paths other than the through path which are produced by the OADM node or the HUB node.
FIG. 23 of the accompanying drawings shows a WDM system 110 including an OADM node. The WDM system 110 is constructed of a transmitting station 111, a receiving station 112, repeaters 113 through 116, and an OADM node 118. The OADM node 118 branches (drops) a WDM signal sent as an optical signal having a particular wavelength from the transmitting station 111 to a tributary which is on a different route from the receiving station 112, and inserts (adds) an optical signal having a particular wavelength from a tributary which is on a different route from the transmitting station 111 into a WDM signal on a through path from the OADM node 118 to the receiving station 112.
According to conventional dispersion management for the WDM system 110, a dispersion map for a longest-distance path P1 (through path) between the terminal stations 111, 112 is optimized, and a dispersion map for a branching path P2 for adding/dropping a signal at the OADM node 118 is not optimized. Therefore, there is developed a deviation between a residual dispersion value in the central channel at the receiving station 112 on the path P1 and a residual dispersion value in the central channel at the OADM node 118 on the branching path P2. Since the eye aperture at a reception point of the OADM node 118 is degraded much more than the eye aperture at a reception point of the receiving station 112, a limitation is posed on the number of wavelengths that can be added/dropped.
FIG. 24 of the accompanying drawings shows a WDM system 120 including a HUB node. The WDM system 120 is constructed of a transmitting station 111, first and second receiving stations 112a, 112b, repeaters 113 through 117, and a HUB node 119. The HUB node 119 has a function to send an optical signal having a particular wavelength, among WDM signals sent from the transmitting station 111, to the first receiving station 112a, and also to send other signals to the second receiving station 112b. After the WDM signals are sent from the transmitting station 111 to the HUB node 119 through a common path, i.e., a path through which all wavelengths (channels) are transmitted in common, the WDM signals are transmitted to the receiving stations 112a, 112b through branched paths.
In the WDM system 120 where the HUB node 119 is present, it has been the conventional practice to perform dispersion compensation optimized for a longest path P1 (it is assumed here that the distance between the HUB node 119 and the receiving station 112a is longer than the distance between the HUB node 119 and the receiving station 112b). Since a dispersion map for a path P3 branched by the HUB node 119 is not optimized, the residual dispersion tolerance of the path P3 is smaller than the residual dispersion tolerance of the path P1, tending to pose a limitation on the number of wavelengths that can be transmitted from the HUB node 119 to the receiving station 112b. 